Multi-stage launch vehicles have been used to deliver payloads into orbit above the Earth and, in some cases, beyond. With such launch vehicles, one or more large and expensive booster stages are used to accelerate a payload up to a velocity sufficient to enter orbit. Typically, after each booster stage has served its intended purpose, it is separated from the remainder of the launch vehicle and discarded. Although this is clearly wasteful, during earlier times the very novelty of space exploration was sufficient to justify the costs. With the increased need and frequency of space launches, the large costs associated with expendable booster stages have become commercially significant.
With many previous multi-stage launch vehicles, recovery of a booster stage was impractical or impossible. Most prior designs store liquid propellant in a single tank. During ascent of these prior launch vehicles, forces due to acceleration direct the liquid propellant to a feed line at the bottom of the tank to supply propellant to the engines. At main engine cut-off (MECO), any liquid propellant remaining in the single tank may disperse, boil off, or otherwise degrade in quality.